Practice question

Digit separation requires selective elimination of interdigital necrotic zones by programmed cell death. BMP ligands through BMPR1A BMPR1B activate phosphorylated Smad1-5-8 cascade, transcriptionally upregulate Msx1 Msx2 and promote caspase-dependent apoptosis with DNA fragmentation. Distal FGFs from digit tips antagonize BMP to protect digit chondrogenesis driven by Sox9. In ducks, Gremlin blocks BMP and retains webbing; ectopic Noggin in chick creates syndactyly. Genetic ablation of BMP2 or BMP4 or Msx2 causes soft-tissue fusion. SHH and FGF10 promote growth, RA modulates proximal identity, but BMP4 execution is central to interdigital apoptosis forming free digits.